The invention relates to ellipsometric systems with an achromatic design for analysing small area of a sample over a wide of wavelengths from ultraviolet (UV) to infrared (IR).
Spectroscopic ellipsometry is a well-known non destructive probe to analyse the properties of a sample (or a layer over a sample). The surface of the sample is illuminated by a luminous beam that is reflected and the polarisation state of the reflected beam (that may be transmitted) is compared to that of the incident beam. This technique allows determining different properties of said sample. These properties may be, for example, the thickness and optical properties of different coatings (single layer or multiple layers) deposited on any substrate.
In conventional ellipsometry, the polarisation vector E of a beam is generally represented by its projections Es and Ep, respectively perpendicular and parallel to the incidence plane. The ratio denoting the change in the polarisation state produced by the interaction of a beam with a surface studied is generally represented by the complex quantity:ρ=tgΨ exp(iΔ)=(Ep/Es)r/(Ep/Es)i
The aim therefore is to measure the independent parameters Ψ and Δ for a given surface.
A description of ellipsometry and ellipsometer systems can be found in the book of AZZAM and BASHARA entitled “Ellipsometry and Polarised Light”, North-Holland, Amsterdam, 1977. Different types of spectroscopic ellipsometers exist in the industry: rotating analyser (ASPNES D. E. et al; Appl. Opt. (1975) 200), rotating polariser, rotating compensator and phase modulated ellipsometer (BOYER G. R. et al. Appl. Opt. 18 (1979) 217).
Due to the high integration in microelectronics and the new developed materials, high sensitivity measurements of optical properties are required.
It is thus highly desirable to obtain a small and compact spot of the focused beam on the sample surface and to have a large range of incidence angle so that the ellipsometer has a high degree of sensitivity for film variations on various substrates. The focused beam on the surface should however show no aberrations in order to fit the microscopic structures.
The current state, of the art ellipsometers employ reflective optics to avoid geometric aberrations and to obtain a continuous incident angular range.
In “Angular scanning mechanism for ellipsometers”, BYRNE D. M. and MAC FARLANE D. L.; Appl. Optics, 30 (1991) 4471, an ellipsometer was proposed in which a pair of stationary concave ellipsoidal mirrors had respectively at their near focus a rotating flat mirror and at their coincident far focus, a sample. By rotating the flat mirror, it was demonstrated a continuously varying angle of incidence on the sample while maintaining a fixed point of incidence. A large range of incidence angle could be achieved with a control on said angle variation only depending on the characteristics of the ellipsoid.
However, since the beam was incident on the mirrors with an angle different from zero, the reflection of said beam introduced a change in the polarisation. These modifications had to be measured and taken into account in the data analysis.
More recently, PIWONKA-CORLE et al. [U.S. Pat. No. 5,910,842] have demonstrated the use of mirrors with elliptical shape surfaces to reduce off-axis aberrations such as <<coma>> in the focused beam.
The described system showed however chromatic aberrations. These aberrations could only be minimised by employing Rochon prisms for the first polariser and the analyser, whose length along the axis of propagation of the light beam was minimised.
The incidence angle range was limited to angles superior to 60° from the normal to the surface to limit the modifications in the beam polarisation upon reflection on the mirrors.
Although, PIWONKA-CORLE et al. tried to minimise them, both previously described systems showed chromatic aberrations.